Optical system for head-mounted display

ABSTRACT

A head-mounted display may include a display system and an optical system in a housing. The display system may have a pixel array that produces light associated with images. The display system may also have a linear polarizer through which light from the pixel array passes and a quarter wave plate through which the light passes after passing through the quarter wave plate. The optical system may be a catadioptric optical system having one or more lens elements. The lens elements may include a plano-convex lens and a plano-concave lens. A partially reflective mirror may be formed on a convex surface of the plano-convex lens. A reflective polarizer may be formed on the planar surface of the plano-convex lens or the concave surface of the plano-concave lens. An additional quarter wave plate may be located between the reflective polarizer and the partially reflective mirror.

This application is a continuation of U.S. patent application Ser. No.16/224,561, filed Dec. 18, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/434,623, filed Feb. 16, 2017, which claims thebenefit of provisional patent application No. 62/383,911, filed on Sep.6, 2016 and provisional patent application No. 62/370,170, filed Aug. 2,2016, all of which are hereby incorporated by reference herein in theirentireties.

BACKGROUND

This relates generally to optical systems and, more particularly, tooptical systems for head-mounted displays.

Head-mounted displays such as virtual reality glasses use lenses todisplay images for a user. A microdisplay may create images for each ofa user's eyes. A lens may be placed between each of the user's eyes anda portion of the microdisplay so that the user may view virtual realitycontent.

If care is not taken, a head-mounted display may be cumbersome andtiring to wear. Optical systems for head-mounted displays may usearrangements of lenses that are bulky and heavy. Extended use of ahead-mounted display with this type of optical system may beuncomfortable.

It would therefore be desirable to be able to provide improvedhead-mounted.

SUMMARY

A head-mounted display may include a display system and an opticalsystem. The display system and optical system may be supported by ahousing that is worn on a user's head. The head-mounted display may usethe display system and optical system to present images to the userwhile the housing is being worn on the user's head.

The display system may have a pixel array that produces image lightassociated with the images. The display system may also have a linearpolarizer through which image light from the pixel array passes and aquarter wave plate through which the light passes after passing throughthe linear polarizer.

The optical system may be a catadioptric optical system having one ormore lens elements formed from clear materials such as glass or plasticand having reflective structures. The lens elements may include aplano-convex lens element and a plano-concave lens element. Theplano-convex lens element may have a convex surface and an opposingplanar surface. The plano-concave lens element may have a concavesurface and an opposing planar surface that faces the planar surface ofthe convex lens element.

A partially reflective mirror may be formed on a convex surface of theplano-convex lens element. A reflective polarizer may be formed on theplanar surface of the plano-convex lens or the concave surface of theplano-concave lens. An additional quarter wave plate may be locatedbetween the reflective polarizer and the partially reflective mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative head-mounted display inaccordance with an embodiment.

FIG. 2 is a diagram of an illustrative head-mounted display showingcomponents of an illustrative optical system in the head-mounted displayin accordance with an embodiment.

FIG. 3 is a diagram of a head-mounted display with another illustrativeoptical system in accordance with an embodiment.

FIGS. 4 and 5 are cross-sectional side views of illustrative lenselements of the type that may be incorporated into a head-mounteddisplay optical system in accordance with an embodiment.

FIGS. 6 and 7 are diagrams of additional illustrative head-mounteddisplays in accordance with embodiments.

FIGS. 8 and 9 are respectively top and side views of lens elements withcylindrical surfaces in accordance with an embodiment.

FIG. 10 is a diagram of an illustrative lens element and reflectivepolarizer during molding operations in a mold in accordance with anembodiment.

DETAILED DESCRIPTION

Head-mounted displays may be used for virtual reality and augmentedreality systems. For example, a pair of virtual reality glasses that isworn on the head of a user may be used to provide a user with virtualreality content.

An illustrative system in which a head-mounted display such as a pair ofvirtual reality glasses is used in providing a user with virtual realitycontent is shown in FIG. 1. As shown in FIG. 1, virtual reality glasses(head-mounted display) 10 may include a display system such as displaysystem 40 that creates images and may have an optical system such asoptical system 20 through which a user (see, e.g., user's eyes 46) mayview the images produced by display system 40 by looking in direction48.

Display system 40 may be based on a liquid crystal display, an organiclight-emitting diode display, an emissive display having an array ofcrystalline semiconductor light-emitting diode dies, and/or displaysbased on other display technologies. Separate left and right displaysmay be included in system 40 for the user's left and right eyes or asingle display may span both eyes.

Visual content (e.g., image data for still and/or moving images) may beprovided to display system (display) 40 using control circuitry 42 thatis mounted in glasses (head-mounted display) 10 and/or control circuitrythat is mounted outside of glasses 10 (e.g., in an associated portableelectronic device, laptop computer, or other computing equipment).Control circuitry 42 may include storage such as hard-disk storage,volatile and non-volatile memory, electrically programmable storage forforming a solid-state drive, and other memory. Control circuitry 42 mayalso include one or more microprocessors, microcontrollers, digitalsignal processors, graphics processors, baseband processors,application-specific integrated circuits, and other processingcircuitry. Communications circuits in circuitry 42 may be used totransmit and receive data (e.g., wirelessly and/or over wired paths).Control circuitry 42 may use display system 40 to display visual contentsuch as virtual reality content (e.g., computer-generated contentassociated with a virtual world), pre-recorded video for a movie orother media, or other images. Illustrative configurations in whichcontrol circuitry 42 provides a user with virtual reality content usingdisplay system 40 may sometimes be described herein as an example. Ingeneral, however, any suitable content may be presented to a user bycontrol circuitry 42 using display system 40 and optical system 20 ofglasses 10.

Input-output devices 44 may be coupled to control circuitry 42.Input-output devices 44 may be used to gather user input from a user,may be used to make measurements on the environment surrounding glasses10, may be used to provide output to a user, and/or may be used tosupply output to external electronic equipment. Input-output devices 44may include buttons, joysticks, keypads, keyboard keys, touch sensors,track pads, displays, touch screen displays, microphones, speakers,light-emitting diodes for providing a user with visual output, sensors(e.g., a force sensors, temperature sensors, magnetic sensor,accelerometers, gyroscopes, and/or other sensors for measuringorientation, position, and/or movement of glasses 10, proximity sensors,capacitive touch sensors, strain gauges, gas sensors, pressure sensors,ambient light sensors, and/or other sensors). If desired, input-outputdevices 44 may include one or more cameras (e.g., cameras for capturingimages of the user's surroundings, cameras for performing gaze detectionoperations by viewing eyes 46, and/or other cameras).

FIG. 2 is a cross-sectional side view of glasses 10 showing how opticalsystem 20 and display system 40 may be supported by head-mounted supportstructures such as housing 12 for glasses 10. Housing 12 may have theshape of a frame for a pair of glasses (e.g., glasses 10 may resembleeyeglasses), may have the shape of a helmet (e.g., glasses 10 may form ahelmet-mounted display), may have the shape of a pair of goggles, or mayhave any other suitable housing shape that allows housing 12 to be wornon the head of a user. Configurations in which housing 12 supportsoptical system 20 and display system 40 in front of a user's eyes (e.g.,eyes 46) as the user is viewing system 20 and display system 40 indirection 48 may sometimes be described herein as an example. Ifdesired, housing 12 may have other suitable configuration.

Housing 12 may be formed from plastic, metal, fiber-composite materialssuch as carbon-fiber materials, wood and other natural materials, glass,other materials, and/or combinations of two or more of these materials.

Input-output devices 44 and control circuitry 42 may be mounted inhousing 12 with optical system 20 and display system 40 and/or portionsof input-output devices 44 and control circuitry 42 may be coupled toglasses 10 using a cable, wireless connection, or other signal paths.

Display system 40 and the optical components of glasses 10 may beconfigured to display images for user 46 using a lightweight and compactarrangement. Optical system 10 may, for example, be based oncatadioptric lenses.

Display system 40 may include a source of images such as pixel array 14.Pixel array 14 may include a two-dimensional array of pixels P thatemits image light (e.g., organic light-emitting diode pixels,light-emitting diode pixels formed from semiconductor dies, liquidcrystal display pixels with a backlight, liquid-crystal-on-siliconpixels with a frontlight, etc.). A polarizer such as linear polarizer 16may be placed in front of pixel array 14 and/or may be laminated topixel array 14 to provide polarized image light. Linear polarizer 16 mayhave a pass axis aligned with the X-axis of FIG. 2 (as an example).Display system 40 may also include a wave plate such as quarter waveplate 18 to provide circularly polarized image light. The fast axis ofquarter wave plate 18 may be aligned at 45 degrees to the pass axis oflinear polarizer 16. Quarter wave plate 18 may be mounted in front ofpolarizer 16 (between polarizer 16 and optical system 20). If desired,quarter wave plate 18 may be attached to polarizer 16 (and display 14).

Optical system 20 may include lens elements such as lens elements 26 and32. Lens element 26 may be a plano-convex lens (lens element) with aconvex surface facing display system 40. Optional lens element 32 may bea plano-concave lens (lens element) with a concave surface S3 facing theuser (eyes 46).

Optical structures such as partially reflective coatings, wave plates,reflective polarizers, linear polarizers, antireflection coatings,and/or other optical components may be incorporated into glasses 10(e.g., system 20, etc.). These optical structures may allow light raysfrom display system 40 to pass through and/or reflect from surfaces inoptical system 20 such as surfaces S1, S2, and S3, thereby providingoptical system 20 with a desired lens power.

Consider, as an example, image light ray R1. As image light ray R1 exitsdisplay 14 and passes through linear polarizer 16, ray R1 becomeslinearly polarized in alignment with the pass axis of linear polarizer16. The pass axis of linear polarizer 16 may be, for example, alignedwith the X-axis of FIG. 2. After passing through polarizer 16, ray R1passes through wave plate 18, which may be a quarter wave plate. As rayR1 passes through quarter wave plate 18, ray R1 becomes circularlypolarized.

A partially reflective mirror (e.g., a metal mirror coating or othermirror coating such as a dielectric multilayer coating with a 50%transmission and a 50% reflection) such as partially reflective mirror22 may be formed on the convex surface of lens element 26. Whencircularly polarized ray R1 strikes partially reflective mirror 22, aportion of ray R1 will pass through partially reflective mirror 22 tobecome reduced-intensity ray R2. Ray R2 will be refracted (partiallyfocused) by the shape of convex surface S1 of lens element 26.

Ray R2 is circularly polarized. A second quarter wave plate such asquarter wave plate 28 may be included in optical system 20 to convertthe circular polarization of ray R2 into linear polarization. Quarterwave plate 28 may, for example, convert circularly polarized ray R2 intoa ray R3 with a linear polarization aligned with the Y-axis of FIG. 2.

Reflective polarizer 30 may be formed adjacent to quarter wave plate 28.With one illustrative configuration, reflective polarizer 30 and quarterwave plate 28 are planar layers and may be formed on the planer surfaceof lens element 26. Reflective polarizer 30 may have orthogonalreflection and pass axes. Light that is polarized parallel to thereflection axis of reflective polarizer 30 will be reflected byreflective polarizer 30. Light that is polarized perpendicular to thereflection axis and therefore parallel to the pass axis of reflectivepolarizer 30 will pass through reflective polarizer 30. In theillustrative arrangement of FIG. 2, reflective polarizer 30 has areflection axis that is aligned with the Y-axis, so ray R3 will reflectfrom reflective polarizer 30 at surface S2 as reflected ray R4.

Reflected ray R4 has a linear polarization aligned with the Y-axis.After passing through quarter wave plate 28, the linear polarization ofray R4 will be converted into circular polarization (i.e., ray R4 willbecome circularly polarized ray R5).

Circularly polarized ray R5 will travel through lens element 26 and aportion of ray R5 will be reflected in the Z direction by the partiallyreflective mirror 22 on the convex surface S1 of lens element 26 asreflected ray R6. The reflection from the curved shape of surface S1provides optical system 20 with additional optical power. At the sametime, the portion of ray R5 that is transmitted by partially reflectivemirror 22 is converted from circularly polarized light to linearlypolarized light by quarter wave plate 18. This linearly polarized lighthas a polarization aligned with the Y axis so that it is absorbed bylinear polarizer 16. As a result, contrast degradation and stray lightartifacts from the transmitted portion of ray R5 are prevented in theimage viewed by the user.

Ray R6 is circularly polarized. After passing back through lens element26 and quarter wave plate 28, ray R6 will become linearly polarized (rayR7), where the linear polarization of ray R7 is aligned with the X-axisof FIG. 2, which is parallel to the pass axis of reflective polarizer30. Accordingly, ray R7 will pass through reflective polarizer 30 toprovide a viewable image to the user.

If desired, glasses 10 may include an additional linear polarizer suchas clean-up linear polarizer 34. Clean-up linear polarizer 34 has a passaxis aligned with the pass axis of reflective polarizer 30 (i.e.,parallel to the X-axis in this example) and will therefore remove anyresidual non-X-axis polarization from ray R7 before ray R7 reachesviewers eye 46.

If desired, an additional lens element such as element 32 with anadditional lens element surface (surface S3) may be incorporated intooptical system 20. Surface S3 may be concave and/or convex and may beused for additional focusing, distortion correction, etc. Element 32 mayhave a planar surface facing lens element 26 and a curved surface (S3)facing viewer 46. Surface S3 may be concave, convex, aspherical,freeform, concave in parts and convex in parts, or may have othersuitable shapes. Curved surfaces in system 20 such as surfaces S1 and/orS3 may be aspherical to improve sharpness or reduce distortion in theimage presented to the user. Lens element 32 may, for example, be placedwith its planar surface adjacent to reflective polarizer 30, quarterwave plate 28, and the planar surface of element 26 (i.e., reflectivepolarizer 30 and quarter wave plate 28 may be sandwiched between theplanar surfaces of lens elements 32 and 26 without an air gap).

Although element 32 provides additional focusing power, optical systemcomplexity and weight may, if desired, be reduced by omitting element32. Moreover, quarter wave plate 28 need not be located on the planarsurface of element 26, but rather may be located at any position betweenpartially reflective mirror 22 and reflective polarizer 30. For example,quarter wave plate 28 may be moved to position 24 between curvedpartially reflective mirror 22 and the convex surface of element 26.

FIG. 3 is a cross-sectional side view of glasses 10 in an illustrativeconfiguration in which optical system includes plano-convex lens element26 and plano-curved lens element 32 (e.g., plano-concave,plano-aspherical, etc.) and in which reflective polarizer 30 is formedon curved surface S3 of lens element 32. Because surface S3 is curved,additional optical power and/or distortion correction capabilities or alarger display field of view may be provided by allowing image light toreflect from reflective polarizer 30 when reflective polarizer 30 hasbeen curved to the shape of surface S3. If desired, quarter wave plate28 may be moved from the position shown in FIG. 3 to a position adjacentto reflective polarizer 30 of FIG. 3 (e.g., in location 50) or may bemoved to position 24 of FIG. 2. The configuration of FIG. 3 is merelyillustrative.

FIG. 4 shows how lens elements 32 and 26 may, if desired, be separatedby an air gap in system 20. Antireflection coatings may be provided onthe planar surfaces of element 32 and/or 26 to reduce reflections, ifdesired.

In the illustrative configuration of FIG. 5, system 20 is formed fromlens elements with additional curved surfaces. In this arrangement,elements 32 and 26 are meniscus lenses and meet at curved mating surface52. The optical systems of FIGS. 4 and 5 may include quarter waveplates, partially reflecting mirrors, and reflective polarizers to formcatadioptric lenses as described in connection with catadioptric lenssystems (lenses) 20 of FIGS. 2 and 3.

In the illustrative configuration of FIG. 6, reflective polarizer 30 hasbeen formed on the surface of additional lens element 54. Reflectivepolarizer 30 and lens element 54 may be attached to the adjacent curvedsurface of lens element 32 using optically clear adhesive (as anexample). If desired, the surface of lens element 54 facing user 46 mayhave a curved surface. The thickness of lens element 54 may, if desired,be constant (e.g., the thickness of element 54 may vary by less than 10%or less than 5% or other suitable amount across its diameter). Inaddition, linear polarizer 34 may be formed on the curved surface oflens element 54 that faces user 46 to help suppress reflections of strayambient light. Linear polarizer 34 may be oriented so that the pass axisof linear polarizer 34 is aligned with the X axis so that rays of imagelight such as light ray R7 of FIG. 2 will pass to user 46 for viewingwhile ambient light rays passing through polarizer 34 (in the −Zdirection) will become X-polarized due to the X-axis pass axisorientation of linear polarizer 34 and will therefore not be reflectedby reflective polarizer 30 (which has a reflective axis oriented withthe Y axis). Obliquely oriented ambient light rays will also tend to bereflected away from user 46 due to the curved surfaces of the lenselements in system 20. The presence of linear polarizer 34 willtherefore help to reduce stray light reflections toward user 46 from theinwardly facing side of system 20.

Outwardly facing surface S4 of lens element 54 may be curved (e.g.,convex) and opposing mating inwardly facing surface S5 of lens element32 may be correspondingly curved (e.g., concave). With one illustrativeconfiguration, surfaces S4 and S5 may be rotationally symmetric aboutthe Z axis of FIG. 6 (e.g., lens elements 54 and 32 may be dome lensesand surfaces S4 and S5 may be dome lens surfaces). This allows lenselement 54 to be rotated relative to lens element 32 (e.g., to alignreflective polarizer 30 to quarter wave plate 28, etc.).

In the example of FIG. 6, surfaces S6 and S7 are planar. This helpsavoid imposing undesired stresses on quarter wave plate 28 (which may,as an example, be formed from a birefringent stretched film). Anotherillustrative arrangement for minimizing quarter wave plate stress isshown in FIG. 7. In the example of FIG. 7, surfaces S6 and S7 have acylindrical curved shape (S6 is convex and S7 is concave so that thesecylindrical shapes mate). Although quarter wave plate 28 of FIG. 7 iscurved, quarter wave plate 28 is only bent (curved) about a single axis(the Y axis) and is not bent about the X axis. As a result, quarter waveplate 28 does not have compound curvature that might impose undesiredstresses on quarter wave plate 28. For comparison, FIGS. 8 and 9 showcross-sectional side views of lens elements 32 and 26 of FIG. 7. FIG. 8is a cross-sectional side view viewed along the Y axis. Quarter waveplate 28 is interposed between cylindrical surface S6 of lens element 32and cylindrical surface S7 of lens element 26 and is bent about an axisparallel to the Y axis as shown in FIG. 8. FIG. 9, which is across-sectional side view of lens elements 32 and 26 of FIG. 7 viewedalong the X axis, shows how surfaces S6 and S7 do not bend about the Xaxis. Because surfaces S6 and S7 have this cylindrical shape, quarterwave plate 28 does not exhibit compound curvature and is not exposed toundesired amounts of stress so that a relatively uniform retardance isprovided by the quarter wave plate 28 across the lens assembly.

FIG. 10 shows how lens elements such as lens element 54 may be formed byinjection molding of plastic (polymer) or other material into mold 56.Reflective polarizer 30 may be placed in mold 56 so that the reflectivesurface of reflective polarizer 30 bears against mold surface S5′ asplastic is injected into the interior cavity of mold 56 to form lenselement 54. Mold surface S5′ can be machined with high accuracy, sopressing reflective polarizer 30 against surface S5′ during moldingoperations will help enhance the smoothness and accuracy of thereflective surface of reflective polarizer 30. Similarly, if desired,reflective polarizer 30 can be formed by molding reflective polarizer 30against opposing surface 58 during injection molding operations.

The lens elements used in optical system 20 may be relatively thin andformed of light-weight materials (e.g., plastic) and/or may be formedfrom materials such as glass. Reductions in weight may help provide user46 with a comfortable viewing experience. It may be easier to mold thelens element(s) with uniform optical properties including lowbirefringence when lens elements such as element 54 have a uniformthickness.

As described in connection with FIGS. 8 and 9, quarter wave plate 28 maybe interposed between lens elements 32 and 26 when elements 32 and 26are bonded together (e.g., using adhesive layers on opposing sides ofquarter wave plate 28). Surfaces S6 and S7 may be planar (e.g., element32 may be a plano-concave element and element 26 may be a plano-convexelement), as described in connection with FIG. 6, or surfaces S6 and S7may be curved (see, e.g., FIG. 7). As described in connection with FIGS.8 and 9, surfaces S6 and S7 can be cylindrical surfaces (surfaces bentaround one axis). In this type of configuration, quarter wave plate 28may bend along only one axis (e.g., quarter wave plate 28 may not haveany compound curves), thereby reducing distortion in quarter wave plate28 and helping to ensure that the retardation provided by quarter waveplate 28 is uniform.

During assembly of optical system 20, a planar piece of quarter wavefilm may be placed between elements 32 and 26 with optical adhesive oneither side of the quarter wave film. Elements 32 and 26 may then beforced together to distribute the adhesive and bend the quarter wavefilm about axis Y (an axis parallel to axis Y). Providing acylindrically curved shape for surfaces S6 and S7 can enable thethickness of lens elements 32 and 26 to be reduced. The use ofcylindrically curved shapes for surfaces S6 and S7 can help make for amore uniform thickness across the lens elements and thereby improve lenselement moldability. When forming injection molded lens elements,uniformity of thickness in the mold cavity can help improve uniformityof flow of the molten plastic as it is being injected into the mold andthe melt front flows across the mold cavity. The presence of a uniformflow during molding can be important for preventing flow lines in themolded lens, particularly when the lens element is thicker at the edgethan the center. More uniform flow can also result in a lowerbirefringence in the molded lens elements. For catadioptric opticalsystems such as system 20, low birefringence in the lens elements helpsto maintain control of the polarization state of the image light, sothat stray light and ghosts are reduced and so user 46 is therebyprovided with a high contrast image without stray light artifacts.Moreover, the cylindrically curved shape of wave plate 28 inconfigurations of the type shown in FIGS. 8 and 9 may help ensure thatlight rays in system 20 pass through quarter wave plate 28 with an anglethat is closer to normal incidence than with planar wave plateconfigurations. As a result, the retardation provided by quarter waveplate 28 may be more uniform across the lens element and the imageprovided to user 46 may as a result be more uniform in contrast withfewer ghost artifacts.

In device 10, image light is converted from unpolarized light tolinearly polarized light, to circularly polarized light, then back tolinearly polarized light, back to circularly polarized light and finallyback to linearly polarized light. For the conversion from linearlypolarized light to circularly polarized light to occur fully so thatpolarization ellipticity is reduced, it may be desirable for quarterwave plates 28 and 18 to be accurately oriented relative to thepolarization axis of the polarizers. For example, it may be desirable toaccurately orient the fast axis of the quarter wave plate 18 at 45degrees to the polarization axis (pass axis) of linear polarizer 16 andthe fast axis of quarter wave plate 28 at 45 degrees to the polarizationaxis (pass axis) of reflective polarizer 30. The fast axes of thequarter wave plates may, for example, be oriented at 45 degrees to thepolarization axes of the respective polarizers within +/−1.5 degrees orother suitable alignment tolerance. Accurate alignment of the quarterwave plates to the polarization axes of the polarizers helps ensure thatlight does not have a mixed polarization state (is not ellipticallypolarized). Accurate alignment therefore prevents portions of the imagelight from following unintended paths that form ghost images thatdegrade contrast and present stray light artifacts.

Linear polarizer 16 and quarter wave plate 18 may be aligned duringlamination. For example, rolls of polarizer film and quarter wave filmcan be accurately aligned to one another in a rewinding process andlaminated together with optically clear adhesive so that the alignmentis maintained. The laminated polarizer/quarter wave film can then beattached to a substrate for mounting into the optical system or attacheddirectly to a cover glass or other structure associated with pixel array14. Emissive displays such as organic light-emitting diode displays andlight-emitting diode displays formed from arrays of crystallinesemiconductor light-emitting diode dies may provide unpolarized imagelight so that attaching a laminated polarizer/quarter wave film to thepixel array allows the display system to emit circularly polarizedlight.

Quarter wave plate 28 can also be accurately aligned with reflectivepolarizer 30. Reflective polarizer 30 can be formed in a curved shape(e.g., by thermoforming with heat and differential pressure or pressureforming), either directly to the concave surface of the lens element 32or to a mold that matches the concave surface of the lens element 32(e.g., mold 56 of FIG. 10). Either the quarter wave plate 28 orreflective polarizer 30 can then be aligned during an assembly processin which two lens elements 32 and 54 are bonded with reflectivepolarizer 30 and curved quarter wave plate 28. Either reflectivepolarizer 30 or the curved quarter wave plate 28 may, for example, bebonded to one of lens elements 32 or 54 and the remaining elements ofsystem 20 may be oriented with a desired alignment accuracy. Apolarimeter can be used to measure through optical system 20 duringalignment to determine how much ellipticity is present and to use thisinformation in guiding the alignment during assembly. In configurationsof the type shown in FIG. 6, the interface between lens elements 32 and26 is planar. In this type of configuration, quarter wave plate 28 canbe bonded to the planar side of plano-convex lens element 26 andreflective polarizer 30 can be bonded to the concave side ofplano-concave lens element 32. Lens elements 32 and 26 can then berotated relative to one another while polarization ellipticity ismeasured. Once it is determined that the quarter wave plate 28 isaligned satisfactorily to reflective polarizer 30, the components ofoptical system 20 can be bonded together to preserve alignment.

If desired, dome optics (lens elements with dome-shaped surfaces) may beused to facilitate alignment of polarizer 30 and quarter wave plate 28.For example, convex surface S4 of element 54 and concave surface S5 ofelement 32 may be dome shaped, allowing these dome lens elements to berotated relative to each other during alignment operations. Quarter waveplate 28 may be bonded between lens elements 32 and 26. Polarizer 30 maybe formed on the surface of lens element 54. Dome lens element 54 maythen be bonded to surface S5 of lens element 32 while aligning polarizer30 and quarter wave plate 28. Dome lens element 54 can be rotated asneeded before bonding to lens element 32 while polarimetric measurementsare made to assess alignment accuracy. If desired, reflective polarizer30 can be molded to surface S4 of lens element 54, as described inconnection with FIG. 10 (e.g., using reflective polarizer 30 as aninsert into mold 56). Applying pressure to the optical plastic forelement 54 during molding forces reflective polarizer 30 against thewall of mold 56 during molding, so that the accuracy and smoothness ofthe reflective surface of reflective polarizer 30 (e.g., the outwardlyfacing surface of reflective polarizer 30) is determined by the accuracyand smoothness of the wall of mold 56, which can be formed to opticalspecifications. After molding, the thickness of molded dome lens 54(e.g. 1 to 3 mm) maintains the surface accuracy of the reflectivesurface of reflective polarizer 30 for ease of handling during assembly.The process of bonding dome lens 54 to the mating dome-shaped surface S5of lens element 32 (e.g. using liquid optically clear adhesive) can thenbe of sufficiently low force that the as-molded surface accuracy of thereflective surface of reflective polarizer 30 on surface S4 of element54 is not degraded.

As shown in FIG. 6, linear polarizer 34 may be formed on the eye side(concave surface S8) of element 54 to help prevent spurious reflectionsof light from the environment. Linear polarizer 34 can be a separatelayer, either flat or curved, positioned between optical system 20 anduser's eye 46. Linear polarizer 34 can also be attached to surface S8 oflens element 54 (e.g., an inner dome lens surface) or can be laminatedto reflective polarizer 30 before polarizer 30 is formed on surface S4of lens element 54. Linear polarizer 34 may be aligned relative toreflective polarizer 30 so the pass axes of the two polarizers arealigned. In this way, linear polarizer 34 absorbs light from theenvironment that has the polarization state that would be reflected byreflective polarizer 30. Light from the environment that has thepolarization state that is transmitted by both linear polarizer 34 andreflective polarizer 30 passes through quarter wave plate 28 and quarterwave plate 18 ending with a linear polarization state that is absorbedby linear polarizer 16. This helps reduce stray light reflections,because reflective polarizer 30 reflects light of this polarizationstate with a high reflectivity, which has the potential to createdistracting reflections of light entering device 10 from behind or tothe side of user 46. At the same time, linear polarizer 34 is aligned sothe transmission axis of the linear polarizer 34 is parallel to thetransmission axis of reflective polarizer 30 and thereby helps enablelinear polarizer 34 to serve as a cleanup polarizer to improve thequality of images from pixel array 14 while reducing the brightness ofthe image light presented to the eye of the user by a relatively smallamount (e.g., <20% if linear polarizer 34 has a transmission of 40% and<10% if linear polarizer 34 has a transmission of 45%). Linear polarizer34 may be a high transmission polarizer with a transmission of at least40%, at least 43%, or at least 45% compared to unpolarized light.

In an embodiment, quarter wave plates 28 in system 20 may be formed frommultiple layers of retarder films laminated together. The layers ofretarder films may be oriented at angles to one another so that togetherthey act as a quarter wave plate with reduced variation in retardationas measured in waves over a broader spectral bandwidth, also known as anachromatic quarter wave. For example, the retardation of quarter waveplates 18 and/or 28 may be within +/−1.5° over a wavelength range of450-650 nm).

A primer (e.g., an adhesion promoting polymer) may be applied to one ormore surfaces of reflective polarizer 30 prior to insert molding of domelens element 54. This may help increase the bond strength betweenreflective polarizer 30 and dome lens element 54 after molding.

Reflective polarizer 30 may, if desired, have a substrate formed from amaterial such as polycarbonate or cyclic olefin that matches the thermalexpansion of the lens elements in system 20 (e.g. acrylate or cyclicolefin lens elements), thereby reducing interfacial stress when opticalsystem 20 is exposed to heat either from display system 40 or theenvironment.

If desired, lens element 26, which is interposed between the other lenselements of system 20 and display system 40 may be made from glass(which may have lower thermal expansion and higher heat resistancecapabilities than plastic) to help resist heat effects from displaysystem 20. In addition, a soft adhesive or an optical grease may be usedto cement quarter wave plate 28 between lens elements 54 and 32 toenable some differential thermal expansion with reduced interfacialstress between the two lens elements and quarter wave plate 28.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A head-mounted display configured to displayimages viewable by a user, comprising: an array of pixels configured toproduce the images; a linear polarizer through which light associatedwith the images passes; a first planar quarter wave plate that receivesthe light from the linear polarizer; a first lens element having a firstsurface that is convex and having an opposing second surface; apartially reflecting mirror on the first surface; a second quarter waveplate at the second surface; a second lens element having a thirdsurface at the second quarter wave plate and an opposing fourth surfacethat is concave; a reflective polarizer at the fourth surface; and athird lens element having a fifth surface and opposing sixth surface,wherein the second lens element is interposed between the first andthird lens elements and wherein the third lens element is a dome lens.2. The head-mounted display defined in claim 1, wherein the fifthsurface is attached to the reflective polarizer.
 3. The head-mounteddisplay defined in claim 1, wherein the linear polarizer is a firstlinear polarizer and wherein the head-mounted display further comprises:a second linear polarizer, wherein the reflective polarizer isinterposed between the second linear polarizer and the second lenselement.
 4. The head-mounted display defined in claim 3, wherein thereflective polarizer is attached to the fifth surface and wherein thesecond linear polarizer is attached to the sixth surface.
 5. Thehead-mounted display defined in claim 3, wherein the second linearpolarizer is interposed between the fifth surface and the reflectivepolarizer.
 6. The head-mounted display defined in claim 5, wherein thefifth surface is interposed between the sixth surface and the secondlinear polarizer.
 7. The head-mounted display defined in claim 3,wherein the second linear polarizer is a separate layer formed betweenthe sixth surface and the user.
 8. The head-mounted display defined inclaim 1, wherein the third lens element has a thickness that varies byless than 10%.
 9. The head-mounted display defined in claim 1, whereinat least one of the fifth surface and the sixth surface is aspheric. 10.The head-mounted display defined in claim 1, wherein the reflectivepolarizer is insert molded onto the third lens element.
 11. Thehead-mounted display defined in claim 1, wherein the reflectivepolarizer includes a primer on one or more surfaces.
 12. Thehead-mounted display defined in claim 1, wherein the third lens elementis attached to the second lens element with optically clear adhesive.13. The head-mounted display defined in claim 1, wherein the third lenselement comprises glass.
 14. The head-mounted display defined in claim1, wherein the third lens element comprises plastic.
 15. Thehead-mounted display defined in claim 1, wherein the third lens elementcomprises more than one material.
 16. A head-mounted display configuredto display images viewable by a user, comprising: an array of pixelsconfigured to produce the images; a first linear polarizer through whichlight associated with the images passes; a first planar quarter waveplate that receives the light from the linear polarizer; a first lenselement having a first surface and an opposing second surface; apartially reflecting mirror on the first surface; a second quarter waveplate at the second surface; a second lens element having a thirdsurface at the second quarter wave plate and an opposing fourth surface;a reflective polarizer at the fourth surface; a third lens elementhaving a fifth surface and opposing sixth surface, wherein the secondlens element is interposed between the first and third lens elements andwherein the third lens element is a dome lens; and a second linearpolarizer, wherein the reflective polarizer is interposed between thesecond linear polarizer and the second lens element.
 17. Thehead-mounted display defined in claim 16, wherein the reflectivepolarizer is attached to the fifth surface and wherein the second linearpolarizer is attached to the sixth surface.
 18. The head-mounted displaydefined in claim 16, wherein the second linear polarizer is interposedbetween the fifth surface and the reflective polarizer and wherein thefifth surface is interposed between the sixth surface and the secondlinear polarizer.
 19. A head-mounted display configured to displayimages viewable by a user, comprising: an array of pixels configured toproduce the images; a first lens element having a first surface that isconvex and having an opposing second surface, wherein the first surfaceis interposed between the array of pixels and the second surface; apartially reflecting mirror on the first surface; a quarter wave plateat the second surface; a second lens element having a third surface atthe quarter wave plate and an opposing fourth surface; a reflectivepolarizer at the fourth surface; and a dome lens having a fifth surfaceand opposing sixth surface, wherein the second lens element isinterposed between the first lens element and the dome lens.
 20. Thehead-mounted display defined in claim 19, wherein the dome lens has athickness that varies by less than 10%.